Electric solenoids have been used to provide a number of functions in automotive applications including, but not limited to idle speed control, exhaust gas recirculation valves, fuel vapor purge valves, and the like. Pneumatic actuators were used prior to electrically controlled solenoids. These solenoids were typically characterized as having either a relatively high force over a relatively short operating stroke, or having a relatively low force over a relatively long operating stroke.
The availability of space in conventional engine compartments has made it necessary to reduce the size of solenoids while maintaining their high force and stroke characteristics. One such application, i.e., that requires reduced packaging, is the solenoid actuator for a variable cam/valve timing mechanism that is used to control the opening and closing of the engine's valves.
In this application, the solenoid is required to control the mechanism over a predefined stroke. At the proximate center of the stroke, the mechanism will not change the cam/valve timing. As the solenoid moves from the proximate center of stroke to one end of the stroke, the mechanism will advance the cam/valve timing. As the solenoid moves from the proximate center of stroke to the opposite end of the stroke, the mechanism will retard the cam/valve timing. After changing the cam/valve timing, the solenoid is returned to the proximate center of stroke until a change to the cam/valve timing is required.
Controlling the cam/valve timing may provide benefits such as but not limited to higher engine power output, lower vehicle tailpipe emissions, higher fuel economy, and the like. However, conventional variable force solenoids have not been completely satisfactory with respect to their stroke and profile characteristics.
The basic construction of a traditional solenoid 10 with a flat-faced armature 12 is shown in FIG. 1, in accordance with the prior art. The other main components of the solenoid include the pole piece 14, coil 16, flux tube 18, and an area defining an air gap 20. The air gap 20 is generally defined as a variable space between the facing surfaces of the armature 12 and the pole piece 14.
With respect to operation, current is first applied to the coil 16 to provide a magnetizing force. The magnetic field created by this magnetizing force then induces magnetic flux throughout the magnetic circuit and across the air gap 20 between the armature 12 and the pole piece 14. Axial force is generated at the air gap 20 due to the attraction of the armature 12 to the pole piece 14. Movement of the armature 14 to close the air gap 20 can do useful work. The force is given by the following formula: F=KA[(NI)2/(AG)2]; wherein K=a constant; A=the armature area; N=the number of turns of the coil; I=the current; and AG=the air gap between the armature and pole piece.
Two problems generally arise if this type of solenoid is used. First, it is desired for the force to be proportional to the current, but is instead proportional to the current squared. Second, the force should be independent of armature position, but instead is proportional to 1/AG2.
Therefore, there exists a need for new and improved variable force solenoids, wherein the solenoids include features such as but not limited to relatively long stroke and low profile characteristics.